This invention relates to data communication systems, especially of the kind that may be used as local area networks or in connecting the elements of data processing systems
A local area network allows stations connected by the network to communicate with one another. One problem which must be overcome in any such network is that of contention between stations which wish to transmit simultaneously. One approach is to allow stations to pass an entitlement to transmit from one to another in a predetermined sequence. This approach, which is sometimes referred to as token-passing, has the advantage that it is deterministic, unlike that used in networks of the Ethernet type. Both token-passing rings and token-passing buses have been proposed, and an examole of each is described, together with the Ethernet type, in "Towards a local network standard" by W. Myers in IEEE Micro, Vol. 2 No. 3, August 1982 at pages 28 to 45.
In a token-passing ring data passes successively through every station of the ring that is active and can be modified as it passes. Entitlement to transmit is therefore passed on as a control code, termed a token, which is common to all stations. A station wishing to transmit user data waits until it sees the token, modifies it so it is not seen by subsequent stations, transmits its user data and then regenerates the token to pass on to the next station in the ring.
In a token-passing bus the stations are connected to a common bus and every transmission from any station is received by all other stations unmodified. Tokens are therefore passed on as explicitly addressed messages. The stations are in consequence made more complicated by the need to generate the required addresses and bandwidth is wasted in transmitting them.
The object of this invention is to provide an entitlement-passing system that does not pass data successively through the active stations of the system and yet avoids the need to pass entitlement by explicitly addressed messages.